This invention relates generally to a method of making a metallic article being coated to have enhanced wear resistance and an increased pitting resistance.
It is well known that lubricated concentrated rolling contacts can fail from surface or subsurface initiated pitting. Sliding contacts can fail from excessive wear, scuffing or seizure. These failure mechanisms are controlled globally by oil film thickness, hertzian contact stresses, and lubrication at asperity contacts. These factors, in conjunction with other factors, determine the distribution of contact stresses near asperities, friction coefficient, and contact flash temperature. All of these factors will influence pitting and wear failures.
Meshing of teeth contact surfaces in gears usually operate in the region of mixed-film lubrication where the film thickness to roughness ratio, lambda, is less than three. This results in the load being shared between the fluid and the asperity contact. The lubrication behavior in this region is influenced by the overall distributions of lubricant film thickness, pressure, shear stress and flash temperatures within the hertzian contact and the local variation of these quantities around the asperity contacts.
The combination of rolling and sliding can initiate fatigue cracks at the surface that will result in pit formation. A high percentage of carbides or nitrides produced at the surface of a component result in greater high temperature strength, resistance to wear, and increased pitting life. Currently, non-oxide boron containing coatings, e.g. boron carbide and boron nitrides, are being used to enhance wear resistance in rolling, sliding, and mixed mode contact surfaces as set forth in U.S. Pat. No. 5,549,764 xe2x80x9cWear Resistant Coated Steel Articlexe2x80x9d which issued Aug. 27, 1996 to Gary L. Biltgen and is assigned to Caterpillar Inc.
However, in actual practice it has become apparent that additional frictional reduction and wear resistance is desirable.
The present invention is directed to overcoming one or more of the problems as set forth above.
The invention may be characterized as a modified boron coated metallic component having improved enhanced wear resistance and increased pitting resistance.
The component comprises a metallic substrate, a first coating (e.g. boron carbide), a second coating (e.g. an mixture of boron carbide and boron oxide) and a third coating(e.g. boron oxide) deposited on the substrate. The first coating has a thickness generally no greater than about 3.0 microns. The second coating has a thickness no greater than 1 micron. The third coating has a thickness generally no greater than about 0.5 microns.
In one aspect of the invention, a coated wear resistant metallic article and method of making the article of this invention comprises a basic article, a first coating material, a second coating, and a third coating. The basic article is a metallic material, preferably steel. The first coating material is a non-oxide boron containing coating, preferably boron carbide. The second coating is a mixture of the non-oxide boron containing coating and boron oxide. The third coating material is boron oxide.
A further aspect of the invention is that the disclosed coatings of the metallic component operate to augment the wear resistance and pitting resistance of the component. This can significantly reduce the possibility of failures of the metallic component.